The Oregon Department of Agriculture has banned the application of two pesticides to linden trees following multiple incidents of bee deaths this summer.

Neither professional applicators nor homeowners may spray lindens or other tilia species with pesticides containing imidacloprid or dinotefuran, two neoniconitoid ingredients that can harm bees.

The rule takes effect immediately, and applies for the next six months.

"Although we took significant steps last year to restrict the use of these pesticide products, we've seen more cases involving bumblebees attracted to blooming linden trees and pesticide applications," ODA Director Katy Coba said.

Over the past week, ODA investigated two mass die-offs of bees around linden trees.

In the first case, at a north Eugene apartment complex, a pesticide applicator sprayed linden trees in bloom, in violation of current labeling restrictions. The company had its license suspended and likely will face civil sanctions.

In the second case, in Beaverton, hundreds of bees were found dead around linden trees that had been sprayed with imidacloprid in May.

"There may not be a violation there," ODA spokesman Bruce Pokarney said. "We're continuing the investigation."

ODA also is investigating the die-offs of four Clackamas County beekeepers' entire colonies. It's uncertain whether those are due to pesticides or another cause.

The rash of deaths comes a year after a highly publicized die-off of 50,000 bumblebees at a Wilsonville shopping center.

That incident led to the formation of a legislative task force on pollinator health that will begin meeting on Monday.

It also led Oregon to require that pesticides containing imidacloprid and dinotefuran sold in the state be labeled with instructions prohibiting use on linden trees and other tilia species.